CoDEmanX wrote:you need to select the pose bones before inserting a keyframe, maybe thats the problem?

Of course, I did it : I select the armature of the bone in pose-mode then I change its position and finally I press the key I and choose LocRot;
But I have noticed that it happend on the last keyframe when it is current .
(the previous one is unfortunately modified too even unselected)